1. Field of the Invention
This invention relates to the art of telemetry as applied particularly to seismographic exploration.
2. Related Prior Art
In seismic exploration, acoustic signals are injected into the earth from a location at or near the surface thereof. The acoustic signals radiate downwardly and are reflected from subsurface formations. The reflected acoustic signals return to the earth's surface where they are detected by a plurality of seismic sensors or sensor groups. The sensors are usually deployed along an assigned line and are spaced tens or hundreds of feet apart. Up to one hundred or more sensors or sensor groups may be distributed over such a line, which may be three miles long.
The detected reflected acoustic signals are transmitted to a central signal-recording system. In accordance with conventional seismographic surveying practice, the sensors are located along the assigned line at increasingly greater distances from the recording system. There is therefore a nearest and a farthest sensor or sensor group relative to the recording system.
Commonly, the plurality of seismic sensors transmit data to the recording system through a like number of physically separate transmission channels. Each channel is scanned according to a predetermined sequence. Because of the high cost of a large number of individual transmission channels, single-channel, time-multiplexed telemetry systems have been proposed, wherein the recording system polls each sensor in sequence and identifies each individual sensor by an address unique to that sensor. Alternatively, various clocking schemes have been proposed whereby each sensor has a unique response to one or more clocking signals emitted by the recording system.
The known telemetry systems are characterized in that each sensor unit must, in some specific manner, be unique and distinguishable from its companion sensors on a per-channel basis. Additionally, in the case of multichannel sensors or arrays, a switching system must be used to change channel assignments as the various sensors are advanced in accordance with the so-called common-depth-point seismic exploration method.
If each one of a large number of sensors must be identified uniquely, there is the possibility of confusion in sensor identification attendant when replacing lost or defective sensors or when changing array configurations.
It would be very advantageous, to avoid confusion, if all seismic sensors or sensor groups were identical and interchangeable.
It is an object of this invention to provide a telemetry system having one time-shared or time-multiplexed channel adapted to transmit signals from a plurality of identical, interchangeable sensors or sensor groups to a signal-recording device.